1. Field of the Invention
The present invention generally relates to electro-optical systems for reading indicia, for example, bar code symbols, having parts with different light reflectivities and, in particular, to an arrangement for, and a method of, focusing a light beam directed to the indicia, or focusing return light reflected from the indicia, by performing relative motion between a pair of prisms through which the light beam or the return light is transmitted.
2. Description of the Related Art
Various electro-optical readers and systems have previously been developed for reading bar code symbols appearing on a label, or on a surface of a target. The bar code symbol itself is a coded pattern of indicia. Generally, the readers electro-optically transform graphic indicia of the symbols into electrical signals which are decoded into alphanumeric characters. The resulting characters describe the target and/or some characteristic of the target with which the symbol is associated. Such characters typically comprise input data to a data processing system for applications in point-of-sale processing, inventory control, article tracking and the like.
The specific arrangement of symbol elements, e.g., bars and spaces, in a symbol defines the characters represented according to a set of rules and definitions specified by a code or symbology. The relative size of the bars and spaces is determined by the type of code used, as is the actual size of the bars and spaces.
To encode a desired sequence of characters, a collection of element arrangements is concatenated to form the complete symbol, with each character being represented by its own corresponding group of elements. In some symbologies, a unique “start” and “stop” character is used to indicate where the symbol begins and ends. A number of different bar code symbologies presently exists. The symbologies include one-dimensional codes such as UPC/EAN, Code 39, Code 128, Codabar, and Interleaved 2 of 5.
In order to increase the amount of data that can be represented or stored on a given amount of symbol surface area, several new symbologies have been developed. One new code standard, Code 49, introduced a two-dimensional concept of stacking rows of elements vertically instead of extending elements horizontally. That is, there are several rows of bar and space patterns, instead of one long row. The structure of Code 49 is described in U.S. Pat. No. 4,794,239. Another two-dimensional code structure known as PDF417 is described in U.S. Pat. No. 5,304,786.
Electro-optical readers have been disclosed, for example, in U.S. Pat. No. 4,251,798; U.S. Pat. No. 4,369,361; U.S. Pat. No. 4,387,297; U.S. Pat. No. 4,409,470, No. 4,760,248 and U.S. Pat. No. 4,896,026, all of which have been assigned to the assignee of the present invention. These readers generally include a light source consisting of a gas laser or semiconductor laser for emitting a light beam. The use of semiconductor devices as the light source in readers is especially desirable because of their small size, low cost and low power requirements. The laser beam is optically modified, typically by a focusing optical assembly, to form a beam spot having a certain size at a focal distance at which a target is located. Preferably, the cross-section of the beam spot at the focal distance approximates the minimum width between symbol regions of different light reflectivity, i.e., the bars and spaces.
In conventional readers, the light beam is directed by a scan component along a light path toward a target symbol. The reader operates by repetitively scanning the light beam in a scan pattern, for example, a line or a series of lines across the target symbol by movement of the scan component such as a mirror disposed in the path of the light beam. The scan component may sweep the beam spot across the symbol, trace a scan line across and beyond the boundaries of the symbol, and/or scan a predetermined field of view.
Readers also include a sensor or photodetector which functions to detect light reflected or scattered from the symbol. The photodetector or sensor is positioned in the reader in an optical path so that it has a field of view which extends at least across and slightly beyond the boundaries of the symbol. A portion of the light beam reflected from the symbol is detected as return light and converted into an analog electrical signal. A digitizer digitizes the analog signal. The digitized signal from the digitizer is then decoded, based upon the specific symbology used for the symbol, into a binary data representation of the data encoded in the symbol. The binary data may then be subsequently decoded into the alphanumeric characters represented by the symbol.
It is known to change the focal distance to increase the depth of focus of the reader and the range of locations at which the target may be located. As described in U.S. Pat. No. 4,808,804, the laser or the focusing lens may be shifted. A plane parallel plate having sections of different thicknesses may be moved into and out of the path of the light beam to change the focal distance.
It is also known to image a symbol using a two-dimensional array such as a charge coupled device (CCD) sensor. Collection optics including a focusing lens is used to focus the return light onto the CCD sensor. As described in U.S. Pat. No. 6,336,587, a bifocal system changes the focal distance between two values by inserting and removing a plane parallel plate radially into and out of the optical path.
Many applications call for a hand-held reader whose arrangement of electro-optical components must be compact, lightweight, structurally robust to withstand rough handling, and energy efficient to increase the working life of an onboard battery. The smaller is the CCD sensor, the shorter is the focal distance. For such miniature optical systems, the thickness of the plane parallel plate becomes necessarily thinner if the range of focal distances is to remain the same. A very thin plate, however, whether made of glass or plastic, on the order of 0.5 mm in some applications, is difficult to manufacture while still maintaining acceptable optical quality. When mounted with either an adhesive or mechanical fasteners, the thin plate sometimes deforms. The strength or rigidity of the plate is insufficient for applications requiring a high resistance to shock forces during expected rough handling.
In addition, the amount of lateral space necessary to radially move the plate into and out of the optical path is relatively large. For example, if the optical aperture at the plate is 3 mm in diameter and a 0.5 mm margin is needed for mechanical tolerance and for avoiding optical irregularity, then the lateral area needed must be at least 8 mm, and this dimension poses a limit on how small the optical system can be miniaturized.